'Closed' compressor designed to beat the heat

Monroe, LA —In response to requests from the medical and controlled atmosphere markets, Thomas Industries engineers set out to design a long-life, quiet-running, oilless, articulating piston compressor. The result is a new product line called the Vortex.

To successfully design this next generation compressor, two major issues had to be overcome: heat dissipation and surface finish of the cylinders. Increasing the amount of heat dissipated meant cooler running equipment, and smoother surfaces would increase the life of the wear components.

The extruded aluminum sleeve is over-molded with cast aluminum to create the cylinder.

Getting the heat out.

Air compressors are used in many applications that cannot tolerate any foreign particles in the air stream, precluding the use of typical coolants such as oil. The major factor controlling the length of a compressor's life is temperature. In fact, a reduction of just a few degrees leads to increased life. A typical 1.5-hp compressor has a head temperature of 255F at 120 psig. Thomas engineers knew that in order to reduce operating temperature, they would have to find a way to get rid of the heat quicker. Traditional compressor cylinders are manufactured from cast iron. But while it has the desired hardness and surface finish characteristics, the problem with cast iron is that it has a relatively low coefficient of thermal conductivity (46.0 W/m-K).

One alternative engineers considered was cast aluminum. But although it has better heat transfer characteristics (231.0 W/m-K) it is simply too soft to be used as is for the compressor's cylinder. To increase its hardness, engineers chose to oxidize the surface of the internal diameter of the cylinder, creating a thin, yet tough surface. During anodization of the cylinder, a 0.001-inch penetration and a 0.001-inch buildup of an oxide layer develops. Use of the more thermally conductive aluminum helped engineers not only reduce the head temperature dramatically (190F), but also allowed them to enclose the crankcase. Since compressors must rely on convective cooling to dissipate the heat, the decible output of the Vortex is in the low 70's compared to the mid 80's for open systems.

Engineers also addressed surface finish since the smoother the bore, or cylinder, the less wear on the piston rings, and the longer the compressor life. The surface finish requirement for the cylinder is 8-12 micro-inches (surface finish of a mirror is 8 micro-inches). Unfortunately, casting grades of aluminum have silicon particles to achieve proper flow characteristics, resulting in a surface finish requiring a secondary operation. To make matters worse, post anodizing operations, such as roller burnishing, boring, and honing, actually damage, or remove the anodized surface on the bore.

"In our process, we insert a MIL spec extruded aluminum sleeve that has a low silicon content [less than 1% compared to more than 8% for standard cast aluminum] into a cylinder design that is then cast with standard die cast aluminum," says Andy Bell, product design engineer. "The benefit here is that we can machine the part and hone the bore to a great surface finish, then anodize the part without losing much of our surface finish. This way, we eliminate any post anodizing processes that may affect the protective anodized layer".

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